Technical Field
The present disclosure relates generally to converters and, more particularly, to control devices and methods for quasi-resonant AC/DC flyback converters.
Description of the Related Art
Converters, and particularly offline drivers of light emitting diode (LED) based lamps for bulb replacement, are often desired to have a power factor greater than 0.9, low total harmonic distortion (THD) and safety isolation. At the same time, for cost reasons, it is desirable to regulate the output DC current generated by such a converter as required for proper LED driving without utilizing a closed feedback loop between a primary side and a secondary side of the converter. In this way, a current sensing element, a voltage reference and an error amplifier on the secondary side, as well as an opto-isolator or optocoupler to transfer the generated error signal from the secondary side to a control circuit on the primary side, are no longer required. This is referred to as opto-less regulation. In addition to opto-less regulation, recently considerable emphasis has been given to the total harmonic distortion (THD) of the ac input current caused by such a converter, and in some geographical areas achieving THD<10% is becoming a market requirement.
High-power-factor (high-PF) flyback converters are able to meet power factor and isolation specifications with a simple and inexpensive power stage. In a high-PF flyback converter, like in any high-PF converter topology, there is no energy reservoir capacitor after an input rectifier bridge that receives an AC mains input voltage. Thus, the voltage output from the rectifier bridge, which is the input voltage to the power stage of the converter, is a rectified sinusoid. To achieve a high-PF and low-THD, the input current to the rectifier bridge must be sinusoidal-like and must track the AC mains input voltage supplied to the rectifier bridge, thus originating a time-dependent input-to-output power flow. As a result, the output current from the rectifier bridge contains a large AC component at twice the frequency of the AC mains input voltage.
A quasi-resonant (QR) flyback converter has a power switch turn-on that is synchronized to the instant a transformer of the converter demagnetizes (i.e. the secondary current has become zero), normally after an appropriate delay. This allows the turn-on to occur in the valley of the drain voltage ringing that follows the demagnetization, which is often termed “valley-switching.” Typically, peak current mode control is used, so the turn-off of the power switch is determined by a current sense signal reaching the value programmed into a control loop that regulates the output voltage or current from the converter.
In markets such as the LED lighting market, the current trend is to provide compact and low cost solutions for converters for driving LEDs, while at the same time maintaining high performance in terms of LED current regulation, power factor PF, distortion THD and efficiency. For example, converters may be contained in products that need to meet specific performance criteria such as those set forth in Energy STAR specifications. In the LED lighting market, these converters are typically QR flyback converters that include analog divider circuitry that is usually a non-negligible portion in terms of silicon area of an integrated circuit containing the converter circuitry. This increases the cost and complexity of such a QR flyback converter. In addition, such a QR flyback converter typically includes line-sensing circuitry to sense the instantaneous rectified AC mains input voltage supplied to the converter. The power loss in such line-sensing circuitry may be, for example, 10 mW-15 mW. Some of the latest market requirements, such as EU COC Ver.5 and US DOE February 2014, specify total power consumption for the entire converter to be lower than 75 mW-100 mW in a no-load condition. As a result, the power loss in the line-sensing circuitry may no longer be considered insignificant or negligible. There is a need for improved QR flyback converter circuits and methods to satisfy current market requirements.